Insight into the Transport Mechanism of Low-Molecular-Weight Charge-Neutral Organics in Reverse Osmosis: the Critical Role of Molecular Polarity

Reverse osmosis is extensively applied for desalination, potable water reuse, and ultrapure water production, but suffers from insufficient rejection of charge-neutral organics with low-molecular-weight (<150 Da), whose transport mechanisms remain poorly understood. In this study, amides and alcohols were identified as the most permeable substances, exhibiting permeation rates 30 to 90 times higher than those of inorganic ions and carboxylic acids. Systematic molecular parameter analysis revealed that molecular polarity, alongside van der Waals volume, determines transport behavior. By incorporating the molecular polarity index into the solution-friction model, it successfully explained transport differences between compounds of similar size but different functional groups. Theoretical calculations revealed the transport mechanism for charge-neutral organics: highly polar organic molecules preferentially partition at the membrane interface through hydrogen bonding, and are subsequently transported across the membrane channels via water clusters. A predictive model was developed, achieving >85% accuracy for diverse charge-neutral organics under environmentally relevant concentrations (μg·L^-1). These findings advance the mechanistic understanding beyond size exclusion effects for RO and guide membrane design optimization for enhanced organic rejection.